There are numerous industrial applications in which it is desirable to form a solid layer of a material on the surface of a substrate. For example, it is desirable to form a conductive metal layer in applications as diverse as the manufacture of printed circuit boards, aerials, and antennae such as those found in mobile telephones, radio frequency identification devices (RFIDs), smart cards, contacts for batteries and power supplies, arrays of contacts for flat screen technologies (liquid crystal displays, light emitting polymer displays and the like), electrodes for biological and electrochemical sensors, smart textiles and decorative features.
In this specification, unless the context requires otherwise, the adjective solid, in the context of a solid layer, or solid substrate, refers to being in the solid (rather than liquid or gas) phase of matter. A solid layer or substrate may be plastic, elastic, resilient, rigid, gelatinous, permeable or have any other property consistent with being solid phase.
In some of these applications, the solid layer that is formed covers the surface. In other applications, the solid layer is patterned, and the accuracy and fineness of detail of the pattern can be important. For example, printed circuit boards may have intricate patterns of copper conductive tracks. Accuracy and fineness of detail is important in determining the extent of miniaturisation possible on such printed circuit boards, and the reliability of the electronic circuits built thereon.
Some methods of forming a solid layer on a surface of a substrate require a catalyst, or other activator. For example, the electroless plating process is a solution chemistry plating technique which has been used for many years to apply a conductive metal coating layer to a substrate surface, which can be flat or shaped. In the electroless process, a substrate is immersed in a succession of baths in turn.
An example of the electroless process used to form a copper layer on the surface of a substrate would be as follows:
Firstly, a plastics substrate is etched in a chromic acid/concentrated sulphuric acid bath at 68±2° C. to microscopically etch the surface of the plastics substrate, ensuring good adhesion of the copper to the surface of the plastics substrate.
Secondly, any hexavalent chromic species left on the plastics substance are neutralised in a bath comprising approximately 30% concentrated hydrochloric acid at around 50° C. The plastics substrate is then added to a third bath in which an activator is added to prepare the plastics substrate surface to absorb the catalyst in the next step. This third bath is typically approximately 30% concentrated hydrochloric acid, at room temperature.
Next, the plastics substrate is dipped into a fourth bath, which includes a dilute solution of a palladium colloid along with tin salts. A colloid deposits on the surface of the plastics material to catalyse the deposition of copper in the subsequent plating steps. This bath includes a high proportion of tin salts, approximately 30% concentrated hydrochloric acid, and is operated at room temperature.
The fifth bath into which the plastics substrate is dipped includes an accelerator which activates the absorbed palladium, improving the speed and uniformity of deposition. Accelerator baths include around 30% concentrated hydrochloric acid.
Finally, the activated plastics substrate is dipped into a sixth bath including a plating solution which, catalysed by the palladium colloid on the plastics substrate, causes copper to deposit onto areas of the plastics substrate which were coated with the catalyst. The plating solution includes a copper salt, formaldehyde as a reducing agent, and sodium hydroxide to activate the formaldehyde. The composition of the plating solution must be carefully temperature controlled, with a temperature of 45±2° C. being appropriate for some commercially applicable compositions.
In the above example chemistry, the catalyst is required for formation of the copper layer, and the acid pre-treatment step is important as it helps the resulting metal layer adhere to the substrate.
Various alternatives to this chemistry are known.
For example, WO 2004/068389 describes a method of forming a conductive metal region on a substrate, comprising depositing on the substrate a solution of a metal ion, and depositing on the substrate a solution of a reducing agent, such that the metal ion and the reducing agent react together in a reaction solution to form a conductive metal region on the substrate. In some embodiments, a catalyst or other activator is required to start the reaction which forms the conductive metal region. In general, a catalyst is applied to a substrate surface, which is then brought into contact with the chemical composition which reacts, catalysed by the catalyst, to deposit a metal on the surface of the substrate.
It is known to deposit on a substrate, e.g by inkjet printing, a catalyst for a metal-forming reaction, with the catalyst applied in a solution containing a polymeric binder. See, for example, WO 02/099162 which discloses use of binders such as ethyl cellulose.
U.S. Pat. No. 6,495,456 discloses formation of electrodes on a chip substrate by a process that involves applying a photo-active catalyst liquid (of unspecified composition) to a chip substrate, irradiating the substrate with light to activate irradiated portions of the liquid (possibly selectively, e.g. using a mask) and then using electroless plating to form metal on the activated portions.
It is known to use ultra violet radiation and other means to reduce palladium acetate deposited on a substrate to palladium metal, followed by electroless plating of copper. Reduction may be performed selectively, by use of a contact mask, to produce patterned catalyst. Alternatively, palladium produced by infra red treatment may be patterned by excimer laser ablation using a metal contact mask. See Zhang et al “VUV light-induced decomposition of palladium acetate films for electroless copper plating” Applied Surface Science 109/110 (1997) 487-492 and Esrom “Fast selective metal deposition on polymers by using IR and excimer VUV photons” Applied Surface Science 168 (2000) 1-4.
U.S. Pat. No. 3,900,320 discloses a process for metallizing a plastic or ceramic base. A pre-plate solution comprising a compound of catalytic metal, such as a palladium salt, binder material such as one or more polymers and solvent are coated on the base and dried so as to form a thin polymer layer of about 20 Angstrom to about 3000 Angstrom thick which may thereafter be directly plated by contact with an electroless plating solution. The pre-plate solution has specified viscosity characteristics and specified high concentration levels of catalytic metal compound. A photosensitive polymer former can be used as a component of the pre-plate solution specifically for photographically developing a plateable pattern on a substrate such as a circuit board, printing plate or the like.